Via/BSM pattern optimization to reduce DC gradients and pin current density on single and multi-chip modules

ABSTRACT

A carrier for an electronic device such as an integrated circuit chip is designed by assigning two different voltage domains to two separate areas of the contact surface of the carrier, while providing a common electrical ground for both voltage domains. The integrated circuit chip may be a microprocessor having a nominal operating voltage, and the different voltages of the two voltage domains are both within the tolerance range of the nominal operating voltage but one of the voltage domains is aligned with a high power density area of the microprocessor (e.g., the microprocessor core) and provides a slightly greater voltage. The higher power voltage domain preferably has a ratio of voltage pins to ground pins that is greater than one.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the design of integratedcircuits and packaging for semiconductor chips, and more particularly toa method of assigning voltage domains (power and ground) to contacts ofa chip carrier or interconnect.

2. Description of the Related Art

Integrated circuits are used for a wide variety of electronicapplications, from simple devices such as wristwatches, to the mostcomplex computer systems. A microelectronic integrated circuit (IC) chipcan generally be thought of as a collection of logic cells withelectrical interconnections between the cells, formed on a semiconductorsubstrate (e.g., silicon). An IC may include a very large number ofcells and require complicated connections between the cells. A cell is agroup of one or more circuit elements such as transistors, capacitors,resistors, inductors, and other basic circuit elements grouped toperform a logic function. Cell types include, for example, core cells,scan cells and input/output (I/O) cells. Each of the cells of an IC mayhave one or more pins, each of which in turn may be connected to one ormore other pins of the IC by wires. The wires connecting the pins of theIC are also formed on the surface of the chip. For more complex designs,there are typically at least four distinct layers of conducting mediaavailable for routing, such as a polysilicon layer and three metallayers (metal-1, metal-2, and metal-3). The polysilicon layer, metal-1,metal-2, and metal-3 are all used for vertical and/or horizontalrouting.

As the size of integrated circuits continues to shrink, and pindensities grow, it becomes increasingly more difficult to interconnectthe chip to external circuitry. Chips are commonly attached to asubstrate, e.g., a printed circuit board (PCB) using a carrier orpackage which fans out the connections to pads or pins on the PCB. FIG.1 illustrates a typical chip assembly which includes an IC chip 1, apackage 2, a PCB 3, and miscellaneous components such as capacitors 4.These various elements may be electrically coupled using surface-mountconnections with C4 solder ball arrays 5. IC chip 1 is connected topackage 2 which is in turn connected to PCB 3. Package 2 and PCB 3 bothhave multiple horizontal layers interconnected by vertical vias. Asingle layer may contain multiple planes, i.e., some for wiring andothers for an electrical ground plane or a power plane. A given plane inpackage 2 may have multiple connections to the top and bottom surfacesto couple ground or power planes of IC chip 1 to ground or power planesof PCB 3. It is common to find 24 or more levels of wiring within apackage.

The package itself can significantly affect the performance of theintegrated circuit it supports, particularly as power supply currents,power densities, and operating frequencies increase. In addition toshrinking feature sizes and advances in lithography in CMOS technologythat have increased circuit integration density (and thereby powerdensity), there has also been a reduction in the chip's operatingnominal voltage. These factors have progressively made it more difficultto deliver clean and controlled power to IC chips, and power delivery isoften the most critical parameter in a system design. Decreasing channelwidths are further leading to an exponential increase is leakagecurrents.

To more efficiently optimize leakage and power consumption on the chip,some chips are being partitioned into multiple voltage domains. However,partitioning in this manner requires the different voltages to becontrolled very tightly which results in major constraints on the chipcarrier design. The parameters of concern include the voltage gradientsavailable at the circuits across the chip and pin current magnitudes atthe carrier/PCB interface, all of which should ideally be minimized.Optimization is even more difficult with complex circuits likemicroprocessor chips that integrate one or more CPU (central processingunit) cores, input/output (I/O) interfaces, memory control units andseveral other functional units into one chip. Each of these buildingblocks are designed to meet their performance targets and are built withappropriate transistor types and circuit densities which causes a highpower density variation across the chip area. In particular, the cores(which have a very high circuit density) typically have a much higherpower demand compared to the rest of the functional units and are moresensitive to DC-drop (gradient) and current density.

Unfortunately, state-of-the-art chip carrier designs for single ormulti-chip modules (SCM/MCM) are typically designed using a homogenouspattern for the bottom-side metallization (BSM) pins with noconsideration given to circuit or power densities. When calculating therequired power vias/pins for each voltage domain, the projected totalcurrent is simply divided by the maximum allowed pin/via current; thechip power density floorplan is not taken into account. As a result, theexpected maximum pin current is higher than the calculated averagecurrent by a factor of 3-4. It would, therefore, be desirable to devisean improved technique for delivering power to an integrated circuit chipwhich could reduce DC gradients across the chip and power dissipation onthe chip carrier. It would be further advantageous if the techniquecould reduce the maximum pin currents at the carrier/PCB interface.

SUMMARY OF THE INVENTION

It is therefore one object of the present invention to provide animproved method of designing a carrier for an integrated circuit chip.

It is another object of the present invention to provide a chip carrierwhose power contacts (voltage/ground) are arranged to more efficientlydeliver power to different voltage domains of a chip.

It is yet another object of the present invention to provide a chipcarrier having power interconnections designed to reduce DC gradientsacross the chip.

The foregoing objects are achieved in a method of designing a carrierfor an electronic device such as an integrated circuit chip, by definingtwo separate areas of the contact surface of the carrier and assigningdifferent voltage domains to the two areas while providing a commonelectrical ground for both voltage domains. In an exemplaryimplementation, the integrated circuit chip is a microprocessor having anominal operating voltage, and the different voltages of the voltagedomains are both within the tolerance range of the nominal operatingvoltage, but one of the voltage domains is aligned with a high powerdensity area of the microprocessor (e.g., the microprocessor core) andprovides a slightly greater voltage. That voltage domain preferably hasa ratio of voltage pins to ground pins that is greater than one. Thecontact surface may be a bottom-side metallization (BSM) of the carrier,and the pins on the BSM are interconnected with pins on the top-sidemetallization (TSM) using vias that extend through the thickness of thecarrier. The pins of the top-side metallization are adapted tointerconnect with contacts of a printed circuit board.

The above as well as additional objectives, features, and advantages ofthe present invention will become apparent in the following detailedwritten description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 is a side elevational view of a conventional integrated circuit(IC) chip assembly wherein an IC package interconnects an IC chip to aprinted circuit board, with the IC package and printed circuit boardshown in cross-section;

FIG. 2 is a plan view of one embodiment of a chip assembly constructedin accordance with the present invention, illustrating placement of anIC chip on a chip carrier having a different voltage domain area alignedwith a high circuit density area of the chip;

FIG. 3 is an elevational cross-section taken along lines 3-3 of FIG. 2;and

FIG. 4 is a plan view of a pin pattern for a contact surface of the chipcarrier of FIG. 2, depicting one implementation of the different voltagedomain wherein the ratio of voltage pins to ground pins in this area isgreater than one.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

With reference now to the figures, and in particular with reference toFIG. 2, there is depicted one embodiment 10 of a chip assemblyconstructed in accordance with the present invention. Chip assembly 10is generally comprised of an integrated circuit (IC) chip 12 and a chipcarrier 14. Chip 12 may be for example a microprocessor having a core16. Chip 12 is supported by chip carrier 14 as indicated by the dashedrectangle 15 which represents the shadow or footprint of chip 12. Themounting surface of chip carrier 14 has a multitude of contacts or pinswhich interconnect with pins of chip 12 using solder ball arrays. Chipassembly 10 may include additional interconnection levels, such as aprinted circuit board (PCB) which is connected to the other side of chipcarrier 14.

Core 16 of microprocessor 12 is a relatively high power density area,that is, it has a much higher power demand compared to the rest of thefunctional units of the microprocessor. Core 16 may also be moresensitive to voltage gradients and current density. Chip carrier 14 isdesigned to complement the layout of core 16 within microprocessor 12 bydividing the contact surface of carrier 14 into at least two separateareas having different voltage domains, i.e., wherein two voltages areindependently provided. In this example, one of the voltage domains 18provides a first voltage V₁, and this voltage domain 18 generallycoincides with the superposed area of core 16 when chip 12 is mounted oncarrier 14. In other words, the chip carrier surface area that isaligned with core 16 is totally assigned to the first voltage domain.The remainder of the contact surface of carrier 14 is in the othervoltage domain 20 which provides a second voltage V₂ to the rest of thepower connections for chip 12. The second power domain 20 may alsoinclude signal vias/pins, but in this embodiment no signal vias/pins aredefined in the first voltage domain 18. Each of the voltage domains 18,20 share a common ground plane (segmented) within carrier 14. While FIG.2 depicts the second voltage domain as extending across the entirecontact surface of carrier 14 (except for the core voltage domain), itneed not be so expansive, as pins outside the chip shadow do notcontribute much to power distribution.

For a microprocessor application, the nominal operating voltage of thechip might be in the range of 0.8-1.2 volts, in which case the voltagedomains of carrier 14 may be adapted to provide voltages of V₁=1.10volts and V₂=1.09 volts. Thus, in this example the smaller area voltagedomain is designated for the greater of the two voltages to service thehigher power density area of the chip and, even though the two voltagesare slightly different, they are still within the tolerance range of thesame nominal voltage.

FIG. 3 is a sectional view of chip assembly 10 taken along lines 3-3 ofFIG. 2, and further illustrates the alignment of pins of carrier 14 forthe first voltage domain 18 with pins of core 16, as well as thealignment of vias extending through the thickness of carrier 14. Firstvoltage domain 18 is surrounded by second voltage domain 20. Thisexample only uses two voltage domains, but additional voltage domainscan be provided according to the voltage/power requirements of theparticular application. Also, the different voltage domain areas in thisexample are adjacent, but the invention could be implemented as wellwith non-contiguous areas.

The present invention accordingly describes a design technique toachieve a stable on-chip power supply across the chip and to reduce themaximum pin currents. A reduction of the power dissipated on the chipcarrier itself is also achieved, which reduces the total power theregulator of the overall system needs to deliver. By increasing thepower under the high power density areas of the chip, and since theground is common between the different voltage domains, the groundvoltage gradient and pin currents are lower than that seen by the highpower voltage domain.

The power pin assignments in the high power density area can be furtheroptimized by making the ratio of the V₁/Ground pins greater than one.Pin assignments for an exemplary implementation are shown in FIG. 4which represents the bottom-side metallization (BSM) or contact surfaceof carrier 14 within the footprint 15 of chip 12. Each square in FIG. 4represents a pin assignment, either V₁, V₂ or ground (G), whichinterconnects with a respective power supply or return pin of IC chip12. The first voltage domain 18 is defined under the high power densityarea of chip 12, i.e., core 16, with only V₁ and ground pins from thetop-side metallizaion (TSM) to the BSM to reduce or minimize DC drop andgradient. The BSM pattern in first voltage domain 18 is a 7×7 pin array,with some rows and columns having alternating V₁ and ground pins whilesome rows and columns have only V₁ pins. In the depicted example theratio of voltage pins to ground pins in first voltage domain 18 is37:12. The ground via/pin assignments not only reduce the maximum pincurrents but also ensure adequate V₁ power supply impedance (reducinginductive loops). The second voltage (lower power density) domain 20 isassigned to the rest of the chip and BSM pins in an alternatingvoltage/ground pattern (within the shadow 15 of chip 12) orvoltage/signal/ground pattern (outside of the shadow 15 of chip 12).This second voltage may be used for the input/output (I/O) power domainin which case these assignments ensure a good signal return path.

The size of chip assembly 10 may vary considerably depending upon theparticular application. In an exemplary embodiment, carrier 14 isadapted to interconnect a chip 12 whose surface area is approximately 10mm×10 mm, with a core footprint size of about 5 mm×5 mm. The carrierthickness is about 2 mm. The pin spacing on the BSM is approximately 1mm, and the pin spacing on the TSM is approximately 0.2 mm.

The foregoing design principle can be applied for different chip carriersubstrates, e.g., multilayer ceramic, multilayer glass ceramic, andorganic (polymeric) chip carriers with or without build-up layers. Ineach of these cases, the present invention provides a significantreduction of the DC gradient across the chip and power dissipation onthe chip carrier, and further reduces the maximum pin currents at thecarrier/PCB interface.

Although the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiments, as well asalternative embodiments of the invention, will become apparent topersons skilled in the art upon reference to the description of theinvention. For example, the invention is described in the context of amicroprocessor chip having a single core, but the invention could beimplented as well in a microprocessor having two or more cores, in whichcase the higher voltage domain would comprise multiple areas, underlyingeach of the cores. It is therefore contemplated that such modificationscan be made without departing from the spirit or scope of the presentinvention as defined in the appended claims.

1. A method of constructing a carrier for an electronic device,comprising: defining a first area of a contact surface of the carrier;defining a second area of the contact surface separate from the firstarea; assigning a first voltage domain to the first area of the contactsurface, adapted to provide a first voltage to the electronic device;assigning a second voltage domain to the second area of the contactsurface, adapted to provide a second voltage which is different from thefirst voltage to the electronic device; and providing a commonelectrical ground for return currents from both the first and secondvoltage domains.
 2. The method of claim 1 wherein: the electronic devicehas a nominal operating voltage; and the first and second voltages areboth within a tolerance range of the nominal operating voltage.
 3. Themethod of claim 1 wherein the first area of the contact surface has aplurality of voltage pins and a plurality of ground pins, and a ratio ofthe voltage pins to the ground pins is greater than one.
 4. The methodof claim 1 wherein the contact surface is a bottom-side metallization,and further comprising: interconnecting pins of the first and secondvoltage domains with pins of a top-side metallization using vias thatextend through a thickness of the carrier, the pins of the top-sidemetallization being adapted to interconnect with contacts of a printedcircuit board.
 5. The method of claim 1 wherein: the first voltage isgreater than the second voltage; and the first area is aligned with afootprint of a high power density area of the electronic device.
 6. Themethod of claim 5 wherein the first area is smaller than the secondarea.
 7. A carrier for an integrated circuit chip comprising: asubstrate having a contact surface with a plurality of pins on thecontact surface, said pins arranged in at least first and second areaswherein both areas have common ground contacts, the first area has firstvoltage contacts for a first voltage domain, and the second area hassecond voltage contacts for a second voltage domain.
 8. The carrier ofclaim 7 wherein: the integrated circuit chip has a nominal operatingvoltage; the first voltage domain has a first voltage within a tolerancerange of the nominal operating voltage; and the second voltage domainhas a second voltage within the tolerance range of the nominal operatingvoltage.
 9. The carrier of claim 7 wherein the first area of saidcontact surface has a plurality of voltage pins and a plurality ofground pins, and a ratio of said voltage pins to said ground pins isgreater than one.
 10. The carrier of claim 7 wherein said contactsurface is a bottom-side metallization, and said substrate further hasvias that extend through a thickness of said substrate to interconnectpins of the first and second voltage domains with pins of a top-sidemetallization, said pins of said top-side metallization being adapted tointerconnect with contacts of a printed circuit board.
 11. The carrierof claim 7 wherein: the first voltage domain has a first voltage whichis greater than a second voltage of the second voltage domain; and thefirst area is aligned with a footprint of a high power density area ofthe integrated circuit chip.
 12. The carrier of claim 11 wherein thefirst area is smaller than the second area.
 13. A chip assemblycomprising: an integrated circuit chip having a high power density areawith a plurality of first power supply pins and a plurality of firstreturn pins, and a low power density area with a plurality of secondpower supply pins and a plurality of second return pins; and a carrierhaving a contact surface, said contact surface having a first area witha plurality of first voltage pins and a plurality of first ground pins,and having a second area with a plurality of second voltage pins and aplurality of second ground pins, wherein: the first area is aligned withthe high power density area, the second area is aligned with the lowpower density area, said first power supply pins are respectivelyconnected to said first voltage pins, said second power supply pins arerespectively connected to said second voltage pins, said first returnpins are respectively connected to said first ground pins, said secondreturn pins are respectively connected to said second ground pins, saidfirst voltage pins are adapted to provide a first voltage, said secondvoltage pins are adapted to provide a second voltage which is differentfrom the first voltage, and said first and second ground pins areconnected to a common ground plane of said carrier.
 14. The chipassembly of claim 13 wherein: said integrated circuit chip has a nominaloperating voltage; the first voltage is within a tolerance range of thenominal operating voltage; and the second voltage is within thetolerance range of the nominal operating voltage.
 15. The chip assemblyof claim 13 wherein a ratio of said first voltage pins to said firstground pins is greater than one.
 16. The chip assembly of claim 13wherein said contact surface is a bottom-side metallization, and saidcarrier further has vias that extend through a thickness of said carrierto interconnect said first and second voltage pins with respective pinsof a top-side metallization, said pins of said top-side metallizationbeing adapted to interconnect with contacts of a printed circuit board.17. The chip assembly of claim 13 wherein the first voltage is greaterthan the second voltage.
 18. The chip assembly of claim 17 wherein thefirst area is smaller than the second area.